A polarimeter can be used to measure changes in polarization caused by optical activity in materials, such as anisotropic crystalline solids and solutions containing chiral molecules.
A conventional polarimeter usually consists of a pair of linear polarizers and a photodetector, such as a photodiode. Light from a monochromatic source passes sequentially though a first polarizer having a fixed orientation, the sample and a second polarizer (or “analyzer”). The photodetector is used to detect the intensity of light reaching it from the analyzer.
Rotating the analyzer relative to the first polarizer varies the intensity of light reaching the photodetector. Thus, in the absence of an optically-active sample, the angle needed to minimize light intensity is 90°. However, in the presence of an optically-active sample, additional rotation, θ, is required.
The need for mechanically rotating the analyzer can be avoided by using an electrically-controllable wave plate (or “retarder”) to introduce a controlled amount of optical rotation and, thus, compensate for optical activity of the sample. Notwithstanding this, it is generally desirable to simplify the polarimeter.
A polarimeter is described “All-electric detection of the polarization state of terahertz laser radiation”, S. D. Ganichev, W. Weber, J. Kiermaier, S, N. Danilov, P. Olbrich, D. Schuh, W. Wegscheider, D. Bougeard, G. Abstreiter, and W. Prettl, journal of Applied Physics, volume 103, page 114504 (2008) in which longitudinal photogalvanic currents are determined by degree of linear polarization.